1. Field of the Invention
The present invention relates to a current excitation type light emitting element. In addition, the present invention relates to a light emitting device and an electronic device having the light emitting element.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light emitting elements using electroluminescence. In a basic structure of such a light emitting element, a substance having a light emitting property is interposed between a pair of electrodes. By applying voltage to this element, light emission can be obtained from the substance having a light emitting property.
Since such a light emitting element is of self light emitting type, it is considered that the light emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as a flat panel display element. In addition, other advantages of such a light emitting element are that the element can be manufactured to be thin and lightweight and the response speed is very high.
Since the light emitting element can be formed into a film shape, surface light emission can be easily obtained by forming a large-area element. This is a feature which is difficult to be obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light emitting element is extremely effective for use as a surface light source applicable to illumination and the like.
Light emitting elements using electroluminescence are classified broadly according to whether they use an organic compound or an inorganic compound as a light emitting substance.
When an organic compound is used as a light emitting substance, electrons and holes are injected into a layer including a light emitting organic compound from a pair of electrodes by voltage application to a light emitting element, so that a current flows therethrough. The electrons and holes (i.e., carriers) are recombined, and thus, the light emitting organic compound becomes in an excited state. The light emitting organic compound returns to a ground state from the excited state, thereby emitting light. Based on this mechanism, such a light emitting element is called a current excitation type light emitting element.
It is to be noted that the excited state of an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such a light emitting element, there are many problems which depend on a material, and in order to solve the problems, improvement of an element structure, development of a material, and the like have been carried out.
For example, Reference 1 describes a light emitting element in which a guest substance is dispersed in the whole organic function layer between electrodes to enhance the possibility of injection and recombination of electrons and holes for light emission. However, Reference 1 does not refer to a lifetime of the light emitting element (Reference 1: Japanese Published Patent Application No. H 6-158038).
Such a light emitting element including a light emitting organic compound can be driven at lower voltage than a light emitting element using a light emitting inorganic compound; however, such a light emitting element including a light emitting organic compound has a problem of having a short lifetime. Thus, a light emitting element with a longer lifetime is desired.